Changed from /etc to /dev for unfrackpath test

On OS X `/etc` is symlinked to `/private/etc` which causes the test to
fail. I changed to use `/dev` which is not symlinked by default but
present on all other Unix and Unix like systems. I also added an
additional informative assert to check if `/dev` is for some reason
also a symlink and fail on that rather than the unfrackpath test.